Robert C. Speed

Oblique collision and tectonic wedging of the South American continent and Caribbean terranes

Our studies of the surface structures, seismicity, and gravity anomalies of the Caribbean-South American plate-boundary zone of northeast Venezuela and Trinidad show that the crustal and upper lithospheric structure and kinematics of this zone are complex and differ markedly along strike. We propose a model of the zone9s lateral transition from near Trinidad west to Caracas, emphasizing three concepts: (1) the steepening, detachment, and then sinking away of Atlantic oceanic lithosphere that is (was) attached to the northern edge of continental South America; (2) the wedging seaward and imbricate thickening of the northern margin of the continent in the region of the detaching Atlantic slab; the continental wedge overrides the slab and is overridden by terranes attached to the Caribbean plate; and (3) that the structural and kinematic transitions are a consequence of the progressive oblique collision between continental South America and the overriding Caribbean terranes.

Tectonic relationships between forearc-basin strata and the accretionary complex at Bath, Barbados

The island of Barbados exposes the crestal zone of the accretionary prism of the Lesser Antilles forearc. Tertiary rocks at Bath, Barbados, include four lithic suites: (1) inter-bedded volcanogenic and mainly calcareous pelagic rocks (the Oceanic beds), (2) radiolarite, (3) siliceous hemipelagic rocks, and (4) melange. Suites 1–3 are in structurally discrete groups of fault packets; melange occurs in apparently diapiric crosscutting bodies. The Oceanic beds occupy a pre-Pleistocene nappe complex that underlies much of Barbados. Radiolarite and hemipelagic rocks belong to a thick accretionary basal complex and constitute the relative autochthon to the Oceanic nappes. Fault rocks of the sub-Oceanic fault zone developed during emplacement of the Oceanic nappes. The lower portion of the tectonic stack is deformed in a major south-verging fold couple with wavelength of ∼250 m. Lower, early emplaced Oceanic nappes are restricted to the core of the major synform and are increasingly deformed with depth. Structures suggest easterly transport of Oceanic nappes and upward younging of nappe emplacement, indicating that the deformation of Oceanic beds propagated westward. The Oceanic nappes provided a seal to rising fluids; transport of nappes may have been facilitated by elevated fluid pressure beneath their sole thrust. Pre- and post-nappe folds record north-south shortening about shallow, west-plunging axes, consistent with folded accretionary strata elsewhere on Barbados. Deformation occurred under shallow nonmetamorphic conditions. The Oceanic beds are interpreted as being trapped outer forearc-basin strata that have been tectonically shouldered by the arcward migrating prism flank or crest. The hemipelagic and radiolarite suites may be offscraped abyssal-plain and trench-wedge accumulations. Paleogene-lower Miocene Oceanic strata were deposited at depths between 2.0 and 4.8 km. Middle Miocene Oceanic beds accumulated at shallower depths (1–1.5 km); thus, early Miocene was the probable time of inception of major uplift and arcward shouldering in the crestal zone of the accretionary prism. Accretion of the basal complex of Barbados probably occurred in late Eocene time.

Geologic and isotopic constraints on the crustal structure of the northern Great Basin

Geophysical data do not define the western limit of North American continental crust in the northern Great Basin because Phanerozoic deformation, particularly Cenozoic extension, has overprinted original gravity, magnetic, and seismic signatures. Stratigraphic and structural analyses of north-central Nevada, however, allow location of the continental-oceanic crustal boundary on the basis of recognition of continental-slope deposits. The stratigraphically defined location of the continental edge corresponds to the position of an isotopically defined boundary between granitic plutons with initial Sr isotopic ratios less than 0.706 to the west and greater than 0.706 to the east. Three generally north-south-trending subprovinces with different Sr and Nd isotopic character can be defined in the northern Great Basin. The boundary between the western and central subprovinces is defined by the 87 Sr/ 86 Sr initial = 0.706 isopleth and corresponds to the stratigraphically defined western edge of transitional North American continental crust. The boundary between the central and eastern subprovinces is defined by the ϵ Nd = -7 line (Farmer and DePaolo, 1983) in north-eastern Nevada and may be entirely within North American crust. A third isotopically defined boundary, imprecisely located in western Utah, marks the western boundary of the largely undeformed North American craton.

Structure of the accretionary complex of Barbados, II: Bissex Hill

This is the second of a series concerning the accretionary complex that is exposed on the island of Barbados and is thought to represent the Lesser Antilles fore-arc at its structural high. Two principle tectonostratigraphic elements are present in the Bissex Hill area: a basal complex composed of east-northeast–striking, generally steeply dipping, fault-bound packages of terrigenous sandstones and mudstones (mostly turbidites), hemipelagic rocks and melange; that complex is overlain by the Bissex Hill nappe, composed of calcareous pelagic rocks, crystalline limestone and foraminiferal arenite. Also present in the basal complex are smaller fault-bound packages, termed “micropackets,” composed of brecciated basal complex lithologies. Basal complex faults are cut by the Bissex Hill nappe, except on the southern margin of the Bissex Hill nappe where melange and hemipelagic lithologies apparently have been faulted over the nappe rocks. This youngest faulting may be driven by Holocene and older melange diapirism. Structural evolution of terrigenous and probably hemipelagic rocks in basal complex fault-packages is characterized by coaxial deformation about southwest-trending axes. Melange packages possibly experienced greater strain as evidenced by girdled fold axes and axial planes and foliations of constant orientation. Packet-bounding faults probably developed during early folding and probably represent surfaces against which packet contents contracted. Micropackets are the result of motions after the assembly of major packets, and are formed through fault imbrication and (or) diapiric injection. Rocks of the Bissex Hill nappe experienced folding first about a north-trending axis, followed by folding about a southwest-trending axis and gentle folding on the entire nappe about an east-northeast–trending axis. Deformation in the basal complex is thought to have occurred by progressive accretion of sediment packets. The Bissex Hill nappe is thought to represent a fore-arc basin complex thrust over the accretionary complex. Quartzose sandstones present in the Bissex Hill nappe that probably had their source in the basal complex indicate that the accretionary complex probably shoaled by the Miocene.

Cretaceous magnetizations in northwestern Nevada and tectonic implications

Paleozoic and Mesozoic arc-related rocks of the Black Rock Desert region of northwestern Nevada have acquired a very stable postfolding secondary magnetization, probably in Late Cretaceous time. The remagnetization, which appears to be of regional extent, probably accompanied emplacement of granitic plutons in a northeast-trending belt across northwestern Nevada near the end of Mesozoic time. The region has experienced no appreciable tectonic rotation since that time.

Candelaria and other left-oblique slip faults of the Candelaria region, Nevada

The region comprising the Candelaria Hills and Excelsior Mountains, Nevada, is structurally anomalous with respect to surrounding tracts of the Basin and Range Province. Its major faults strike east, and some, if not all of them, have undergone significant left slip. Ratios of lateral to vertical displacement components on strands of such faults vary from 2 to 6.5. The region of left-oblique slip faulting seems to constitute the intermediate leg of a gigantic Z pattern of Basin and Range faults. One of these faults, the Candelaria fault system, has been studied closely in pursuit of kinematic details that might characterize the regional faulting. The Candelaria system consists of three en echelon faults of 5 to 10 km trace length joined by connector zones that possess compressional features. The amount of lateral slip transferred from one en echelon fault to another is uncertain; analysis of a complex of faults in one connector zone indicates at least some degree of integrated motion. The same connector zone is the site of a bulge that has an uplift estimated to have been 275 m since 2.8 m.y. B.P. On the basis of fault-plane striae, the extension direction is ∼ N82°W. We propose that the Candelaria fault system and a major asymmetric trough in its northern wall were created in Oligocene time by a regional left-lateral shear system oriented in the extension direction. The connector zones may be kinematic impediments which, at shallow levels at least, have prevented the system from maturing into a single plane fault. The existence of the bulge and the estimated rate of slip on part of the Candelaria fault system suggest that the system is still active.

Synorogenic Quartz Sandstone in the Jurassic Mobile Belt of Western Nevada: Boyer Ranch Formation

The province of Lower Mesozoic layered rocks in western Nevada contains a diversity of marine lithologies whose deposition continued locally as late as Middle Jurassic time. At places in the province, mature quartz sandstone constitutes all or part of the highest stratigraphic units in sections of non-volcanic rocks and is believed to record the last deposition of terrigenous sediments before complete effacement of the marine basin by widespread orogeny. The sandstone was deposited early in the erogenic episode and at least locally in troughs created by folding of subjacent rocks. The sands are anomalously mature with respect to co-deposited clastic components and to coarse detrital materials in nearly all earlier Mesozoic rocks of the province. The problems are the source of the quartz sand and the reasons the sand was deposited synorogenically. The name, Boyer Ranch Formation, is formally proposed for a lithesome of homogeneous Jurassic quartz sandstone and basal conglomerate and limestone in the Dixie Valley region which is approximately in the northern third of the outcrop area of Jurassic quartz sandstone in western Nevada. The Boyer Ranch Formation contains up to 500 ft of sandstone, largely fine-grained calcareous quartz arenite, whose granulometric properties suggest eolian sorting, but whose bedding indicates quiet-water deposition. The sandstone lies above limestone and carbonate-pebble conglomerate with interstitial quartz sand. The inferred early Mesozoic geographic and tectonic histories in the Dixie Valley region suggest that the sands of the Boyer Ranch Formation accumulated at the eastern shoreline of the Mesozoic basin in western Nevada in late Early or Middle Jurassic time. Until the onset of orogeny, the sands remained unlithified, probably owing to eolian saltation, and followed a generally westerly regression of the shoreline. Postulated strong wave action prevented seaward movement of the sand. The late Early Jurassic or Middle Jurassic (or both) orogeny created local troughs which the sea reinvaded and provided an irregularly configured and low-energy shoreline environment such that movement of the sand into the water was no longer impeded. The sands may have evolved locally through the action of water and wind at the beach of the Early Mesozoic sea in western Nevada, or they may have been largely co-derived from a distant source with sands in Jurassic rocks of the eastern Cordillera and the Colorado Plateau.

Slow subduction of old lithosphere in the lesser antilles

Abstract The Lesser Antilles subduction zone is an extreme case of the subduction of old (~ 90 m.y.) lithosphere at a slow (~ 2 cm/y) convergence rate. Focal mechanisms of the largest earthquakes in the area have been obtained using body and surface wave data. During the time period (1950–1978) studied the subduction seismicity appears to represent primarily intraplate rather than interplate deformation. All three large (magnitude seven) earthquakes were from intraplate normal faults; no large thrust faulting earthquakes and few small ones occurred. These observations suggest that the plate boundary is largely decoupled, that subduction is at least partially aseismic, and that the downgoing slab is in a state of extension.

A preliminary paleomagnetic pole for mid-Cretaceous rocks from Tobago: further evidence for large clockwise rotations in the Caribbean-South American plate boundary zone

Abstract The island of Tobago occupies the eastern end of the central (igneous) belt of the Caribbean-South American plate boundary zone. Volcaniclastic sediment of the Albian (∼ 100 Ma) Tobago Volcanic Group and dikes of similar age within it were sampled in two homoclinal sections with different attitudes. The mean of virtual geomagnetic poles for 12 sites (25.4°N, 24.1°E, A95 = 4.2°) is well defined, pre-tilting and apparently reliable, yet is far removed from a similar age reference pole for South America. Five other paleomagnetic studies of Cretaceous rocks from widely separated localities farther west in the plate boundary zone yield pole positions that are remarkably similar to the Tobago pole. Poles obtained from the Guajira Peninsula of Colombia, the islands of Aruba and Bonaire, and the Caribbean Mountains of Venezuela are among those that agree with the Tobago result. The paleolatitudes for study areas within the plate boundary zone are consistent with an origin on or near South America, yet the poles throughout the zone are rotated roughly 90°. Dextral relative motion between the Caribbean and South American plates was probably responsible for the rotation.

Structural development during flysch basin collapse: the fencemaker allochthon, East Range, Nevada

Abstract During Mesozoic deformation in north-central Nevada, a thick succession of Triassic siliciclastic and hemipelagic carbonate strata that were deposited in a deep-water basin shortened against and overrode penecontemporaneous shallow-water carbonates and interbedded siliciclastics at the eastern margin of the basin. Initial rheologic contrasts developed between lithologic units during shallow pretectonic burial localized displacements within fine-grained, poorly-cemented units and along lithologic boundaries. Changes in theology during low-pressure and low-temperature deformation led to more brittle conditions, possibly due to fluid expulsion, and subsequent displacements truncated bedding. Structures in Triassic strata suggest that the W-facing slope of the shelf-to-basin transition localized thrusting. Collapse of the basinal strata against strata of the basin margin and formation of a thrust at the shelf-to-basin transition demonstrate the ability of stratigraphic declivities and associated rheologic contrasts to buttress deformation and localize thrusting.